An organic EL device is a spontaneous light emitting device which utilizes the principle that a fluorescent substance emits light by energy of recombination of holes injected from an anode and electrons injected from a cathode when an electric field is applied. Since an organic EL device of the laminate type driven under a low electric voltage was reported by C. W. Tang et al. of Eastman Kodak Company (C. W. Tang and S. A. Vanslyke, Applied Physics Letters, Volume 51, Pages 913, 1987 or the like), many studies have been conducted on organic EL devices using organic materials as the constituent materials. Tang et al. used tris(8-hydroxyquinolinol aluminum) for a light emitting layer and a triphenyldiamine derivative for a hole transporting layer. Advantages of the laminate structure are that the efficiency of hole injection into the light emitting layer can be increased, that the efficiency of forming exciton which are formed by blocking and recombining electrons injected from the cathode can be increased, and that exciton formed within the light emitting layer can be enclosed. As described above, for the structure of the organic EL device, a two-layered structure having a hole transporting (injecting) layer and an electron transporting light emitting layer and a three-layered structure having a hole transporting (injecting) layer, a light emitting layer, and an electron transporting (injecting) layer are well known. To increase the efficiency of recombination of injected holes and electrons in the devices of the laminate type, the structure of the device and the process for forming the device have been studied.
Further, as the light emitting material, chelate complexes such as tris(8-quinolinolato)aluminum complexes, coumarine derivatives, tetraphenylbutadiene derivatives, bisstyrylarylene derivatives, and oxadiazole derivatives are known. It is reported that light in the visible region ranging from blue light to red light can be obtained by using these light emitting materials, and development of a device exhibiting color images is expected.
Further, in recent years, a large number of investigations have been conducted on the use of a phosphorescent compound as a light emitting material and the use of energy in a triplet state in EL light emission. A group of Princeton University has reported that an organic EL device using an iridium complex as a light emitting material shows high luminous efficiency. In addition to the organic EL device using a low molecular weight material as described above, an organic EL device using a conjugated polymer has been reported by a group of Cambridge University. In this report, light emission has been confirmed from a monolayer of polyphenylene vinylene (PPV) formed in a coating system.
Recent advances in organic EL device are remarkable, and characteristics of the organic EL device allow formation of a thin and lightweight light-emitting device with high luminance under application of a low voltage, wide range of emission wavelengths, and high-speed response, thereby suggesting the possibility of extensive uses.
In association with the significant progress of an organic light emitting device, performance requested of a light emitting material has been growing. For example, Patent Documents 1 to 6 each disclose an anthracene derivative. In addition, Patent Documents 7 to 10 each disclose an anthracene derivative containing dibenzofuran, dibenzothiophene, and a carbazole derivative. Each of those material systems shows improved performance, but involves, for example, the following problem: each of the systems still has low luminous efficiency. Accordingly, none of the systems has satisfied characteristics requested of a light emitting material needed to have an optical output with additionally high luminance or additionally high efficiency with which a voltage applied to the material is converted into light yet. In addition, a sufficient material, especially light emitting material, for an organic EL device having the following characteristics has not been found yet: the material meets requests for durability against, for example, a change over time due to the long-term use of an organic EL device containing the material or the deterioration of the device due to an atmospheric gas containing oxygen, moisture, or the like and for high-efficiency blue light emission taking the application of the device to, for example, a full-color display into consideration.
[Patent Document 1] US 2005/0089717 A
[Patent Document 2] U.S. Pat. No. 7,056,601
[Patent Document 3] WO 02/038524
[Patent Document 4] WO 2005/054162
[Patent Document 5] WO 2005/061656
[Patent Document 6] WO 2004/018587
[Patent Document 7] WO 04/053018
[Patent Document 8] JP 2005-314239 A
[Patent Document 9] JP 2007-063501 A
[Patent Document 10] WO 2005/113531